1. Field of the Invention
The present invention relates to a method and related apparatus for improving Hybrid Automatic Repeat Request (HARQ) operation for a network in a wireless communications system, and more particularly, to a method and related apparatus for modifying a radio resource control message for improving HARQ operation.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS (Quality of Service) requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. Through the 3G mobile telecommunications system, a user can utilize a wireless communications device, such as a mobile phone, to realize real-time video communications, conference calls, real-time games, online music broadcasts, and email sending/receiving. However, these functions rely on fast, instantaneous transmission. Thus, targeting the third generation mobile telecommunication technology, the prior art provides High Speed Package Access (HSPA) technology, which includes High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA), to increase bandwidth utility rate and package data processing efficiency to improve uplink/downlink transmission rate.
HSDPA and HSUPA adopt Hybrid Automatic Repeat Request (HARQ) technology to enhance retransmission rate and reduce transmission delay. HARQ is a technology combining Feed-forward Error Correction (FEC) and ARQ methods, which makes a Universal Terrestrial Radio Access Network (UTRAN) trigger retransmission according to positive/negative acknowledgement signals (ACK/NACK) transmitted from a user equipment (UE), and the UE stores useful information about the last failed transmission for later use. Simply, when a previous data is unsuccessfully decoded, the UE shall combine the received data with the previous data and perform a decoding procedure on the combined data, for enhancing the probability of successful decoding. The procedure of combing the received data with the previous data is called HARQ combining.
To further increase the downlink data rate, 3GPP introduces a Multi-Input Multi-Output (MIMO) technology, with which a UE and a base station, known as a Node-B, utilize multiple antennas to transmit/receive radio signals. The MIMO technology can be further integrated with spatial multiplexing, beam forming and spatial diversity technologies to reduce signal interference and increase channel capacity. When the UE is not configured in the MIMO operation, the UE can only receive one transport block on a High Speed Physical Downlink Shared Channel (HS-PDSCH) in each Transmission Time Interval (TTI), and the UE includes 8 HARQ processes at most. On the other hand, when the UE is configured in the MIMO operation, the UE receives two transport blocks on the HS-PDSCH in each TTI, and the number of HARQ processes is restricted to 12, 14 or 16.
To control the HARQ operation and the MIMO operation of the UE, the UTRAN can set configuration of the MIMO operation in radio resource control (RRC) messages with information elements (IEs) in a RRC protocol layer, and send the RRC messages to the UE through corresponding RRC procedures. For the HARQ operation, the UE is configured to the HARQ operation according to an IE “HARQ Info”. An IE “Number of Processes” included in the IE “HARQ Info” is used for setting the number of HARQ processes used by the UE. For the MIMO operation, the UE is configured to the MIMO operation by receiving an IE “MIMO parameters” included in the RRC message. The IE “MIMO parameters” includes an IE “MIMO operation”, an IE “MIMO N_cqi_typeA/M_cqi ratio” and an IE “MIMO pilot configuration”. The IE “MIMO operation” is used to indicate the UE to activate or continue the MIMO operation, and correspondingly includes two parameter values, which represent “start” and “continue”, respectively. The IE “MIMO N_cqi_typeA/M_cqi ratio” includes information about the antenna numbers of the UE and the Node-B. The IE “MIMO pilot configuration” includes information about diversity, channelization code, etc. Note that, when the IE “MIMO parameters” is not included in the received RRC message, the UE shall stop the MIMO operation.
On the other hand, when the IE “MIMO parameters” is included in the received RRC message, the UE stores the value of the IE “MIMO N_cqi_typeA/M_cqi ratio” and the value of the IE “MIMO pilot configuration” in a MIMO_PARAMS variable and sets the value of a MIMO_STATUS variable. After specific conditions are all satisfied, the UE sets the MIMO_STATUS variable to TRUE and is configured to the MIMO operation. The specific conditions are as follows:
1. The UE is in a CELL_DCH state.
2. The IE “MIMO N_cqi_typeA/M_cqi ratio” is included in the MIMO_PARAMS variable.
3. The IE “MIMO pilot configuration” is included in the MIMO_PARAMS variable.
Note that, HARQ combining is not permitted during transition between MIMO mode and non-MIMO mode. The UE regards the first transmission after the transition as a new transmission, for avoiding incorrect HARQ combining. As mentioned previously, the number of HARQ processes in MIMO mode is restricted to 12, 14 or 16. If the UE does not change HARQ configuration accordingly when the UE performs the mode transition of the MIMO operation, the number of HARQ processes may be incorrect. It means that the IE “HARQ Info” has to be carried in the RRC message which triggers the mode transition of the MIMO operation. In other words, the IE “HARQ Info” and the IE “MIMO parameters” should be both included in the same RRC message. For example, the IE “HARQ Info” and the IE “MIMO parameters” are both included in a CELL UPDATE CONFIRM message. When the UE receives the CELL UPDATE CONFIRM message, the UE can perform the mode transition of MIMO operation and change HARQ configuration according to the CELL UPDATE CONFIRM message so that the number of HARQ processes is changed to the correct number.
In many RRC messages that the UTRAN sends to the UE, a PHYSICAL CHANNEL RECONFIGURATION message is used by the UTRAN to assign, replace or release a physical channel used by the UE. The PHYSICAL CHANNEL RECONFIGURATION message comprises the IE “MIMO parameters” but does not comprise the IE “HARQ Info”. In other words, even if the UE can trigger the mode transition of MIMO operation according to the PHYSICAL CHANNEL RECONFIGURATION message, the UE cannot be configured to the MIMO operation because the number of HARQ processes is not changed according to the PHYSICAL CHANNEL RECONFIGURATION message. As a result, the UE may receive an incorrect number of HARQ processes, and unknown behavior occurs.